Tungsten filaments used primarily in halogen and incandescent lamps are coiled coils, i.e. the tungsten filament or wire is coiled once around a primary mandrel to form the primary coil or the primary set of coils, then that set of coils itself is coiled (the xe2x80x9csecondary set of coilsxe2x80x9d). In the development of a new lamp, it was found that the secondary set of coils of the tungsten filament would become very distorted after the lamp was operated for only a minute. The distortion was so severe that the vertical legs of this filament actually shorted out against the secondary set of coils. Shorting out only one secondary coil reduces the life of the product to approximately 20% of the design life for a filament with only six secondary coils. In order to achieve design life, the coil dimensions needed to be stabilized.
A known method for achieving coil dimensional stability in a tungsten filament with vertical legs is to screw the completed filament (with no primary mandrel in the coil) on a mandrel form, clamp the vertical legs in place, and heat the filament in a vacuum furnace for 2 to 10 hours. After this process, a tungsten plug is inserted into the primary set of coils"" leg to aid in welding the filament to molybdenum foil. For high wattage coils with large wire diameters this is possible. In fine wire coils, high shrinkage (losses of coils) would be incurred due to handling. Furthermore, with both large and fine coils, this process is both labor and time intensive.
Thus, there is a need for a cost effective method of stabilizing the coil dimensions of a tungsten filament with vertical legs by thermally stress relieving the coil prior to mounting in a lamp. This process is useful to prevent the filament from distorting and shorting out the secondary set of coils when the lamp is operated.
A method of dimensionally stabilizing a tungsten filament prior to installation in a lamp comprising the steps of providing a tungsten filament which has been manipulated into a desired shape around a primary mandrel to provide a primary coil, inducing a current in the primary coil, and maintaining the current until the tungsten filament at least partially recrystallizes.